1. Field of the Invention
The present invention relates to a switching power supply unit and more particularly to the switching power supply unit and its control circuit which has an overcurrent detection circuit by using an ON resistor of a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and an overcurrent detection circuit by using an external current detection resistor.
2. Description of the Related Art
In an overcurrent detection circuit employed in the conventional switching power supply unit, it is known that the overcurrent detection circuit configured to detect an overcurrent by an external current detection resistor and the overcurrent detection circuit configured to detect an overcurrent by an ON resistor of the MOSFET are used separately in their proper way depending on their applications. The external current detection resistor is externally mounted in series on the MOSFET. In the case where an overcurrent is detected by using the external current detection resistor, since temperature dependence of a resistance value of the external current detection resistor is small, a special contrivance against temperature changes is not required as the overcurrent detection circuit, however, in the case where an overcurrent is detected by using the ON resistor of the MOSFET, since the ON resistor of the MOSFET exhibits very large temperature dependence, a temperature-compensated overcurrent detection circuit is necessary.
A conventional overcurrent detection method is described by referring to drawings.
FIG. 9 shows a conventional switching power supply unit (hereinafter called a “first DC/DC converter circuit”) in which an overcurrent detection circuit for detecting an overcurrent by using an external current detection resistor is employed.
The first DC/DC converter circuit includes an input voltage source VIN, a control circuit 14, an n-channel high-side MOSFET 2, an external current detection resistor Rs, an n-channel low-side MOSFET 3, an inductor 8, a smoothing capacitor 9, load 10, and the like. The control circuit 14 is further made up of a current detection circuit 4, an overcurrent detection comparator 5, a gate driving circuit 6, a reference voltage source for an overcurrent detection threshold VTH2, and the like.
A negative terminal of the input voltage source VIN is connected to GND (Ground terminal); a positive terminal of the input voltage source VIN is connected to a drain terminal of the high-side MOSFET 2; a source terminal of the high-side MOSFET 2 is connected to one terminal of the external current resistor Rs; the other terminal of the external current resistor Rs is connected to a drain terminal of the low-side MOSFET 3; and a source terminal of the low-side MOSFET 3 is connected to GND. A connection point between the other terminal of the external current resistor Rs and the drain terminal of the low-side MOSFET 3 is connected to one terminal of an inductor 8; the other terminal of the inductor 8 is connected to one terminal of a load 10; and the other terminal of the load 10 is connected to GND. One terminal of the load 10 is connected to one terminal of the smoothing capacitor 9, and the other terminal of the smoothing capacitor 9 is connected to GND. Moreover, RON shows an ON resistor of the high-side MOSFET 2.
The control circuit 14 has an HD terminal, an SW terminal, an LD terminal, and a CS terminal, in which the HD terminal is connected to a gate terminal of the high-side MOSFET 2, the LD terminal is connected to a gate terminal of the low-side MOSFET 3, the SW terminal is connected to a connection point between another terminal of the external current detection resistor Rs and the drain terminal of the low-side MOSFET 3, and the CS terminal is connected to a connection point between the source terminal of the high-side MOSFET 2 and one terminal of the external current detection resistor Rs.
In the current detection circuit 4, to a non-reversal input terminal of an operational amplifier mounted therein is connected to the CS terminal and to its reversal input terminal is connected to the SW terminal and its output terminal is connected to an non-reversal input terminal of the overcurrent detection comparator 5. Further, to a reversal input terminal of the overcurrent detection comparator 5 is connected the reference voltage source for the overcurrent detection threshold value VTH, and an output terminal of the overcurrent detection comparator is connected to an input terminal of the gate driving circuit 6. One output terminal of the gate driving circuit 6 is connected through the HD terminal to the gate terminal of the high-side MOSFET 2, and the other output terminal of the gate driving circuit 6 is connected through the LD terminal to the gate terminal of the low-side MOSFET 3. The external current detection resistor Rs, current detection circuit 4, overcurrent detection comparator 5, and reference voltage source for the overcurrent detection threshold VTH2 make up the overcurrent detection circuit.
The first DC/DC converter circuit, by alternately performing switching between the high-side MOSFET 2 and low-side MOSFET 3, both connected between a positive terminal of the input voltage source VIN and GND, supplies power through the inductor 8 to the load 10.
The external current resistor Rs converts a current IDH flowing through the high-side MOSFET 2 into a voltage ΔVCS, and the current detection circuit 4 made up of the operational amplifier and the like produces and outputs a current detection signal VSNS based on the voltage ΔVCS.
The overcurrent detection comparator 5 compares the current detection signal VSNS with an overcurrent detection threshold value VTH2, and, when the current detection signal VSNS becomes higher than the overcurrent detection threshold value VTH2, outputs an OFF trigger signal to the gate driving circuit 6.
The gate driving circuit 6, by receiving the OFF trigger signal and outputting a low-level gate signal HDRV to the high-side MOSFET 2, turns off the high-side MOSFET 2.
On the other hand, FIG. 10 shows another conventional switching power supply unit (hereinafter, called a “second DC/DC converter circuit”) in which an overcurrent detection circuit for detecting an overcurrent by using an ON resistor RON of the high-side MOSFET 2 is employed. In the second DC/DC converter circuit, instead of the external current detection resistor Rs for detecting the current IDH flowing through the high-side MOSFET 2, the ON resistor RON of the high-side MOSFET 2 is used, and, further, as the overcurrent detection threshold value to be used in the control circuit 15, a threshold value VTH1 corresponding to the ON resistor RON is used instead of the above-mentioned overcurrent detection threshold value VTH2. The ON resistor RON of the high-side MOSFET 2, overcurrent detection circuit 4, overcurrent detection comparator 5, and reference voltage source for the overcurrent detection threshold value VTH1 make up the overcurrent detection circuit. In this case, the CS terminal is connected to a connection point between the positive terminal of the input voltage source VIN and the drain terminal of the high-side MOSFET 2. Here, there is a problem that, the ON resistor RON exhibits positive temperature dependence of about +3000 ppm/□ to +6000 ppm/□, and, therefore, the overcurrent detection level depends on temperatures. To solve the problem of the temperature dependence of a resistance value of the ON resistor RON, a technology is disclosed in, for example, Japanese Patent Application Laid-open No. 2008-187847 (Patent Reference 1) in which changes in resistance value of the ON resistor RON of the high-side MOSFET 2 caused by changes of ambient temperatures are cancelled by letting the overcurrent detection threshold value VTH1 used in the control circuit 15 exhibit positive temperature dependence being equivalent to that of the ON resistor RON, and, as a result, the temperature dependence of the resistance value of the ON resistor RON is offset.
Patent Document 1 refers to Japanese Patent Application No. JP-A-2008-187847